1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the same. In particular, it relates to a semiconductor device such as MOS, BiCMOS and bipolar transistor which includes a vertical type bipolar transistor therein as an electrostatic discharge protection device (ESD protection device) and the method for manufacturing the same.
2. Description of the Related Art
Various type of semiconductor devices providing a protective circuit for the device have been developed.
(i) As an example, Japanese Unexamined Patent Publication Sho 62 (1987)-69678 discloses a horizontal type bipolar transistor as an electrostatic discharge protection device which is formed without using an epitaxial layer in the manufacturing process for MOS transistor. In the horizontal bipolar transistor, an avalanche breakdown current is used as a trigger and an electric current flows by snap-back. A method for manufacturing the horizontal bipolar transistor is described as follows.
As shown in FIG. 94, a thermal oxide film and a nitride film are formed on an entire P.sup.- substrate 301 which serves as a base and has a concentration of about 2.times.10.sup.15 cm.sup.-3, and then the surface of the substrate is etched back so as to leave a nitride film 302 in a region used for an active region.
Subsequently, as shown in FIG. 95, the substrate is oxidized by LOCOS method and .sup.49 BF.sub.2.sup.+ ions are implanted to a desired region for forming a base contact diffusion region by using a resist as a mask 303.
Then, .sup.75 As.sup.+ ions are implanted to a desired region for forming an emitter and collector by using a resist as a mask 304 as shown in FIG. 96. The emitter and the base are shorted with each other by using a metal wiring (represented by the reference numeral 305 in FIG. 98) as shown in FIG. 97, thereby obtaining the horizontal type bipolar transistor.
FIG. 98 shows a plan view of the semiconductor device of this type. When a reverse bias is applied to the collector in the horizontal bipolar transistor of the electrostatic discharge protection device, avalanche breakdown occurs between the collector and the base. The avalanche breakdown current triggers to function a transistor and induces a snap-back, whereby an electric current flows. If a semiconductor device employs the horizontal bipolar transistor in an input part of the circuit, excessive current and voltage caused by static electricity are allowed to escape from the circuit, so that the inside circuit can be protected.
(ii) Alternatively, in addition to the MOS process described above, another device comprising a horizontal bipolar transistor as an electrostatic discharge protection device has been developed in which snap-back occurs by using a punch-through breakdown as a trigger.
In this type of device, as shown in FIG. 99, a thermal oxide film and a nitride film are formed on a P.sup.- substrate 311, and then the nitride film formed on a device isolation formation region is removed to leave a nitride film 312 in a region used for an active region. The distance X between the emitter and collector formed therein is arranged to be shorter than that of the first example described above in order to induce punch-through breakdown between the collector and emitter at a voltage lower than that of inducing avalanche breakdown between the collector and base.
Subsequently, the substrate is oxidized by LOCOS method and the nitride film 312 is removed as shown in FIG. 100. Then .sup.49 BF.sub.2.sup.+ ions are implanted to a desired region for forming a base contact diffusion region by using a resist as a mask 313.
After that, .sup.75 As.sup.+ ions are implanted to a desired region for forming an emitter and collector by using a resist as a mask 314 as shown in FIG. 101.
The emitter and the base are shorted with each other by using a metal wiring (represented by 315 in FIG. 103) as shown in FIG. 102, thereby obtaining the horizontal type bipolar transistor.
FIG. 103 shows a plan view of the semiconductor device of this type. When a reverse bias is applied to the collector in the horizontal bipolar transistor of the electrostatic discharge protection device, punch-through breakdown occurs between the collector and the emitter. The punch-through breakdown current triggers to function a transistor and induces a snap-back, whereby an electric current flows. If a semiconductor device employs the horizontal type bipolar transistor in an input part of the circuit, the excessive current and voltage caused by static electricity are allowed to escape from the circuit, so that the inside circuit can be protected.
(iii) Further, Japanese Patent Publication Sho 61 (1986)-36711 discloses a vertical type bipolar transistor as an electrostatic discharge protection device, in which an avalanche breakdown current is used as a trigger and induces snap-back.
As shown in FIG. 104, spin-on-glass (SOG) containing antimony (Sb), not shown, is applied to a P type substrate 321 to form an N.sup.+ buried collector 323 by driving in, and the SOG on the substrate 321 is removed.
Next, as shown in FIG. 105, an N epitaxial layer 324 is grown on the substrate 321 and the surface of the substrate 321 is oxidized by thermal treatment to form an SiO.sub.2 film 325. After etching SiO.sub.2 325 by using a resist as a mask 326 for forming P well to be used as a device isolation region, .sup.11 B.sup.+ ions are implanted.
FIG. 106 shows that a P well 327 is formed by conducting P well thermal diffusion for forming an device isolation region and then a resist 328 is applied and patterned by photolithographic process, and that the oxide film 325 in the base formation region is etched and .sup.11 B.sup.+ ions are implanted.
FIG. 107 shows that a resist 329 is applied on the substrate 321 on which the base 331 is formed and then an oxide film 330 on the emitter and collector contact diffusion region of the bipolar transistor is etched and .sup.75 As.sup.+ ions are implanted. At this time, the diffusion region bridging from the base to the collector region of the N epitaxial layer 324 is simultaneously formed.
FIG. 108 shows that a contact for forming an electrode is formed after forming an emitter 332, collector contact diffusion region 333 and diffusion region 334.
Then, Al--Si electrode 335 is formed as shown in FIG. 109 such that the emitter and the base are shorted with each other as shown in FIG. 110.
FIG. 111 shows an impurity concentration profile beneath the emitter.
The bipolar transistor used as the electrostatic discharge protection device has an emitter concentration of about 2.times.10.sup.20 cm.sup.-3, base concentration of about 5.times.10.sup.17 cm.sup.-3 and collector N.sup.- epitaxial concentration of about 5.times.10.sup.15 cm.sup.-3. When the reverse bias is applied to the device, avalanche breakdown occurs at about 6 V in a region Y (FIG. 109) near the surface of p-n junction between the collector and the base. The avalanche breakdown current triggers to function a transistor and induces a snap-back as shown in FIG. 112 As the base concentration is 5.times.10.sup.17 cm-.sup.3, punch-through does not occur between the collector and the emitter. Accordingly, if a semiconductor device employs the vertical type transistor in an input part of the circuit, excessive current and voltage caused by static electricity are allowed to escape from the circuit, so that the inside circuit can be protected.
(iv) In BiCMOS process, the horizontal and vertical bipolar transistors are formed in the MOS and bipolar portions, respectively to form a desired BiCMOS.
However, when excessive current flows in the horizontal bipolar transistor of the MOS processes (i) and (ii) which is used as the electrostatic discharge protection device, the instantaneous excessive energy applied to the transistor can not be absorbed sufficiently, so that the p-n junction in the inside circuit or the gate oxide film of the MOS transistor may be broken. This phenomena is induced because the junction in the device is so shallow owing to the horizontal arrangement of the horizontal bipolar transistor that the area through which the electric current passes is limited. Therefore, it is necessary to enlarge the area of the device in order to provide sufficient device ability.
When the vertical bipolar transistor is used as the electrostatic discharge protection device of the bipolar process (iii), the device is in the state of avalanche breakdown until transistor function and snap-back occur. The region having the avalanche breakdown is limited to the diffusion edge where the electric field is concentrated and a current density is very high. Accordingly, the transistor is heated up at the diffusion edge and the device may be broken.